A safety braking system ensures the desired braking force is produced in an extremely reliable manner.
In the field of rail transport, there are principally two types of braking operation: service braking and emergency braking.
The service braking operation is the one which is most commonly used during operation. It can be adjusted between a minimum force value close to 0 and a maximum force value. It can be broken down itself into a plurality of modes, depending on the trains: purely electric brake, purely mechanical brake or combined electric and mechanical brake. It carries out all the “normal” stopping and deceleration operations of the train, and the braking operations for holding on declines. However, it is not secure in the sense that it involves a large number of electric, electronic, mechanical, pneumatic or hydraulic components which may malfunction and therefore bring about a different braking force from that desired, or even, with the new traction chains having traction/static braking commutation, a traction force.
The emergency braking operation is used, as its name suggests, only in the event of an emergency. This emergency condition may be brought about either by an external emergency situation or by a malfunction of the service brake. The object of this brake is to stop the train as quickly and safely as possible. This brake cannot be adjusted but is reliable, that is to say, its probability of failure must be extremely low. This brake must therefore use the fewest possible number of components. Generally, it is purely mechanical, but this requires the mechanical brake to be sized accordingly which may be found to be prohibitive in terms of cost or mass, in particular in a high-speed train where the levels of braking energy to be dissipated are significant. For this reason, it may be very advantageous to produce an electric safety brake.
An electric safety braking device is described in patent DE 101 60 612 A1. The disadvantage of this device is that the force/speed characteristic of the electric safety braking obtained is dependent only on the characteristics of the motor and the brake resistance value selected; it cannot therefore be adjusted, that is to say, it does not allow the desired force/speed curve to be approached. For example, it may lead to excessive levels of force at high speed which would bring about an excessive level of adhesion or, on the contrary, levels of force which are too low at low speed.
German patent application published as DE 10 2004 032 680 A1 relates to a braking system comprising a network of brake resistors which is connected in the form of a star or a triangle and which is capable of being coupled to a permanent magnet motor using an electromechanical type commutator comprising a set of relays. It involves adding a capacitive network which comprises three capacitors which are mounted in parallel on the brake resistor network. In the absence of control by an active regulation system, this capacitive network allows the braking force produced by the electric safety brake at high speed to be increased by compensating for the reactive energy produced by the internal coils of the stator of the motor.